drm/radeon/evergreen: Move 'cik_*()'s prototypes to shared header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/cik.c:6413:6: warning: no previous prototype for ‘cik_init_cp_pg_table’ [-Wmissing-prototypes]
 6413 | void cik_init_cp_pg_table(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/cik.c:6670:5: warning: no previous prototype for ‘cik_get_csb_size’ [-Wmissing-prototypes]
 6670 | u32 cik_get_csb_size(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/cik.c:6702:6: warning: no previous prototype for ‘cik_get_csb_buffer’ [-Wmissing-prototypes]
 6702 | void cik_get_csb_buffer(struct radeon_device *rdev, volatile u32 *buffer)
 | ^~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>